Safety at Sea Seminar Seaworthiness and Safety Yacht Design 102 Paul H. Miller Dept of Naval...

Safety at Sea Seminar

Seaworthiness and SafetySeaworthiness and SafetyYacht Design 102Yacht Design 102

Paul H. MillerPaul H. MillerDept of Naval Architecture, Ocean and Dept of Naval Architecture, Ocean and

Marine EngineeringMarine Engineering

US Naval AcademyUS Naval Academy


Seaworthiness: A Definition“To be

seaworthy, a vessel must be able to defend itself against the perils of the sea…”

A classic example of a “seaworthy design.”


The Big Picture: “Design” in Context

1993 Marine Board Casualty

(of vessels) Study

12% Design and Construction Issues

88% Owner and Operator Issues!


Owner and Operator Issues

• Training• Practice!• Education:Safety at Sea Seminars!

• Maintenance (Inspect, Maintain, Repair)

• Crew Fatigue• Motion? (Bigger is better)• Work load? (Smaller is better!)


Owner and Operator Issues Example:1998 Singlehanded Farallones Race

32 Miles Out into the Pacific and Back

Fifteen (exciting) Minutes after 79 Boats Started


• 1 Hour Later

• 25-35 knot winds

• 12-20 foot waves

• 1/3 Dropped Out

• 1 Vessel Required USCG Support

• Swan 47!

• Flooded

• Hatches Improperly Secured!


• First to Finish

• F/27 Trimaran

• Ave Speed 9.8 knots

• First On Handicap

• 28’ Herreshoff Rozinante Ketch

• Ave Speed 7.2 knots

The Moral of the StoryBoth boats were easy to sail, reef, and steer!


Design and Construction:Three Basic Rules

(Sail or Power)

1. Keep Water Out of the Vessel!

2. Be Able to Direct the Vessel’s Course!

3. Keep the Vessel Upright!

These requirements often conflict with other goals, such as speed or “roominess”!


1. Keeping the Water Out

• Structure• Wood, Fiberglass, Carbon, Metal, (even

Ferro-Cement!) are all acceptable, if sufficient Factors of Safety and quality ($?) construction are used

• Reputation of Designer and Builder are a clue, “standards” are another

• Thoroughly Survey for Current Condition Prior to Each Passage (“PreFloat”)


1. Keeping the Water Out (2)

• Structural “Negative Indicators”• Leaks around through-hulls, hull-deck joint,

keel bolts• Noticeable flexing of hull panels that cause

joinerwork or furniture to move, doors jammed

• Broken joints (tabbing)• Stress cracks


Stress Cracks...

OneAustralia


1. Keeping the Water Out (3)

• Watertight Integrity• Hatches sufficiently strong, bedded and

lockable (from inside and out)• Hatchboards sufficiently strong and

securable (ditto)• Redundant bailing capacity


American Bureau of Shipping Guide for Offshore Racing Yachts

The Elements of Boat Strength by Dave Gerr

Structures References


2. Directing

the Vessel’s Course:Power

and Control


2. Directing the Vessel’s Course (2)

• Rudder and Keel• Sufficient area• Keep attached!

• Weak links• Shaft to tiller/quadrant• Shaft to blade• Cable, sheaves (Think

Simple! Tiller?)


Suggested Equipment References

US SAILING:Safety Recommendations for Offshore Sailing

or

Safety Recommendations for Cruising Sailboats


3. Keep the Vessel Upright - Stability

• Wind and waves are trying to tip the boat over

• Ballast, hull shape and crew weight are trying to keep it uprightDon’t rely on crew weight!


3. Static Stability

• Buoyancy Force acts upward through the center of submerged volume

• Weight Force acts downward through the Center of Gravity

B

W

Sum of the Forces equals Zero!


3. Static Stability When Heeled

B

W

Horizontal distance between Center of Gravity and Buoyancy is Righting Arm (RA)

Righting Moment= Righting Arm x Boat Weight= “Stability”W

“Give me a lever and I will move the earth!”


3. Static Stability When Really Heeled!

B

WW

Moral: A Low Center of Gravity is Nice!


3. Static Stability - Beam Effects

B B

WW

Righting Arms!

W

“Form Stability”“Ballast Stability”


3. Static Stability Lessons• Beam Provides Stability at Small Angles (<40)

• A Low Center of Gravity Always Provides Stability

• For the Same Initial Stability a Narrow Boats Needs a Lower CG (More Keel Ballast-More Weight?)

RM

Heel Angle0 90 180135

Positive Righting Moment - Boat Will Return Upright

Negative RMBoat Will CapsizeLimit of Positive Stability


3. Static Stability Curves

RM

Heel Angle0 90 180135

Narrow Boat - Ballast Stability

Beamy Boat - Form Stability

With Water Sloshing Inside!


3. Dynamic Stability• A Vessel’s Response

to Gusts and Breakers is a function of:• Static Stability (RA x

Boat Weight)• Roll Mass Moment of

Inertia• Surface Area• Roll Damping• Luck!

Uh Oh!


3. Dynamic Stability• Roll Mass Moment of Inertia

• = Sum of (weights x “distances from CG”2)!• Separate weights vertically (preferably lower

so as to get more RM!)

• Surface Area• Reduce Contact Area - dodgers, sails, etc.

• Roll Damping• Keel Area Down Low (Bulbs?)


3. Dynamic Stability

“Ballast Stability” vessels tend to follow gravity!

“Form Stability vessels tend to follow the water surface!”


3. Stability Suggestions

• Capsize Screening Formula (a rough guide)

2

64)( 3

1

lbsBoatWeight

Beam

• LPS >130 for cruisers (may be available from US Sailing for a sistership, or ask a yacht designer)


3. Effect of Size• The Static Stability

Curve, Damping and Mass Moment of Inertia terms do not have “length” factors.

• Displacement, draft, submerged area, beam, and center of gravity are more important!

Size is relative!


“Desirable and Undesirable

Characteristics of Offshore Yachts”

by the Technical Committee of the Cruising Club of

America

ComprehensiveReferences


“Seaworthiness: The Forgotten

Factor”

C. A. Marchaj


Rules To Remember When Selecting, Modifying or Inspecting Your Boat

1. Keep Water Out of the Vessel!2. Be Able to Direct the

Vessel’s Course!3. Keep the Vessel Upright!

If in doubt, talk to an expert!

Safety at Sea Seminar Seaworthiness and Safety Yacht Design 102 Paul H. Miller Dept of Naval...

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